Field of the Invention
The present invention relates generally to the field of orthodontics. More specifically, the present invention discloses a tooth positioning appliance with curved interconnecting elements.
Statement of the Problem
A wide variety of orthodontic aligners have been used for many years in repositioning teeth during orthodontic treatment. It should be noted that the terms “aligner”, “positioner” and “tooth positioning appliance” are largely synonymous as used in the orthodontic field.
This type of orthodontic treatment typically involves separate tooth positioning appliances for the upper and lower teeth. The tooth positioning appliances fit over the teeth, covering nearly all of the facial and lingual surfaces, and also most of the occlusal, or biting surfaces of the teeth. The early positioners described in the prior art were made from a set of plaster models derived from three-dimensional negative dental impressions of the patient's teeth. The plaster dental models were modified by cutting the teeth apart using a small jeweler's saw or rotary cutting discs and repositioning the plaster teeth in a better, straighter, desired arrangement, and holding the teeth in the new arrangement by using dental wax. The reset teeth molds provide the basis for manufacturing the positioners. The resilience of the material from which the positioner is made provides the energy to move the teeth from their original position toward the new straightened position. From the earliest disclosure of the tooth positioner, many of the proposed designs in the prior art have shown moving the teeth in a series of incremental steps. Making a series of appliances is difficult if the tooth arrangement for each step must be made by hand using plaster and wax.
Starting in the early 1990's, digital technologies have begun to provide orthodontists with fundamentally new tools for delivering orthodontic treatment by fabricating tooth models in small but accurate incremental steps. Commercially-available CAD/CAM software can produce the desired tooth models, from which a progressive series of appliances can be manufactured. These tools include 3D imaging of the patient's dentition, and CAD/CAM (computer-aided design and manufacturing) systems for creating virtual models in orthodontic treatment to then produce customized orthodontic appliances.
An example of the successful orthodontic application of these digital technologies is seen in the commercial service known as the Invisalign® program by Align Technology, Inc. of San Jose, Calif. The Invisalign program is largely based on U.S. Pat. No. 5,975,893 (Chishti et al.) and many related patents, including U.S. Pat. No. 6,398,548 (Muhammad et al.). Invisalign tooth positioners are a progressive series of thin, transparent, U-shaped plastic appliances formed over computer-generated forming patterns grown from a virtual model of the patient's dental anatomy. The process for forming aligners uses a combination of vacuum, pressure and heat. This forming process is informally referred to within the orthodontic laboratory community as the “suck down” process.
In order to produce a series of Invisalign-type tooth aligners, a technician first scans a patient's upper and lower model set to obtain CAD-manipulatable virtual models of a patient's dental anatomy. A model set normally consists of one upper and one lower plaster model of the teeth, palate and gums. Once the virtual model of the original malocclusion has been obtained, a technician will then undertake steps involving extensive manipulation of the virtual malocclusion. This involves extensive repositioning of the teeth according to a comprehensive and sequential procedure, ultimately arriving at a finished or ideal occlusion for that patient. The finished occlusion in the virtual model is consistent with the complete repositioning of the patient's upper and lower occlusion that would result at the end of successful conventional orthodontic treatment.
After the steps described above are accomplished, the technician possesses two versions of the patient's teeth available within the virtual CAD environment. One version represents the original malocclusion and the other represents the ideal occlusion. In other words, the technician has the beginning and the end states.
The next step in the Invisalign process involves the creation of an incremental, progressive series of physical forming models. Each of these forming models represents a snapshot of the patient's future occlusion at specific incremental steps along the patient's proposed treatment sequence between the beginning and the end conditions as described above. To accomplish this, the technician creates a virtual “first transition model” that sees a slight repositioning of all or most of the teeth. This first transition model sees some or all of the teeth being subtly moved from their original pre-treatment positions to a virtual first transition position that is in the direction of their intended finished positions. Similarly, a second virtual transition model is created that sees the virtual teeth being moved again slightly further in the desired directions. The objective of the Invisalign technician is to create a series of progressive models, each biased slightly further than the previous one, and each moving the teeth slightly closer to their finished target positions. A final forming model will take the teeth from the series of transition positions and move them into their final, desired positions.
Once such a series of virtual intermediate forming models has been created and a final forming model has been created by the Invisalign technician, the digital code representing each of the models in the series is directed to operate a computer numerically-controlled (CNC) machine known as a rapid prototyping machine. Within a rapid prototyping machine, the series of physical forming models are grown using any of number of conventional processes, such as stereo lithography or 3D printing. The growing step results in the production of hard, physical duplicates of each of the series of virtual intermediate models and the final model.
The next step of the Invisalign process sees each of the series of physical models being in turn mounted in a suck-down machine where a combination of pressure, heat and vacuum is used to form the actual series of progressive aligners from plastic sheet material of a constant thickness. Once the series of progressive aligners are formed and trimmed, they are sequentially labeled, packaged and shipped to the attending orthodontist. The orthodontist then schedules an appointment for the patient, at which time the aligners and instructions for their use are given to the patient. The patient is instructed to wear the first set of aligners for a period of time, typically two weeks. After that, the first set is discarded and the patient transitions to the next set of the series and so on.
The aligners serve to urge the patient's teeth to move according to the positional biases created virtually by the Invisalign technician. The teeth are progressively biased and urged to move in desired directions toward their predetermined finished positions by the resilience of the polymeric material of the aligner. In response to the gentle but continuous forces delivered by the aligners, certain physiological processes involving the creation and resorbtion of the bone supporting the roots of the teeth are initiated. The net result is the slow, progressive orthodontic movement of the roots of the teeth through the underlying bone toward desirable positions and orientations.
Physiologic processes occur when forces are applied to teeth, resulting in bone resorption and new bone apposition. Studies have shown that the most rapid tooth movement occurs when light gentle continuous forces are applied to teeth. Conventional aligner appliances tend to apply heavier forces when the appliance is first placed on the teeth, and the forces decay away fairly rapidly after the appliance has been in place for a day or so. The reason for making many stages of aligners corresponding to very small incremental movements is to keep the forces lighter, and to re-establish the force when it decays by going on to the next aligner stage. Although in principle the use of many stages of aligners should allow the delivery of lighter more continuous forces, in practice there are still problems with providing adequate tooth engagement and with keeping force delivery within the desired physiological range.
Many conventional removable aligners are limited by their design and the mechanical properties of the clear thermoplastic materials that are currently utilized. The clear polymeric materials make the aligner nearly invisible, and that is a great advantage over fixed stainless steel hardware and metal braces. On the other hand, conventional polymeric materials used in forming aligners have a very limited ability to flex. This is particularly a problem when aligning teeth that are not fairly well lined up in the beginning of treatment.
Even when very small movements during each stage are attempted, the appliance may fail to properly engage teeth that need to be moved because the appliance is not adequately flexible and is not designed to allow movement within the plane of the material. If a particular aligner fails to properly engage a tooth, then that tooth will not move to the proper place to engage the next successive aligner in the series. The only present solutions available when aligners fail to properly engage a tooth are: (1) reduce the amount of movement attempted for that particular stage; or (2) place a larger bonded attachment on the tooth. Both of these solutions require reworking the computerized treatment plan. If the plan is not revised, with each successive stage of the appliance, the fit of the aligners deteriorates, and after just a few stages, it becomes obvious that the teeth are not moving according to the original computerized treatment plan, forcing a revision of the treatment plan.
Solution to the Problem
The present invention seeks to overcome the limitations of the lack of flexibility of the appliance material by providing a tooth-clasping element for each tooth that is connected by curved interconnecting elements to the tooth-clasping elements of nearby teeth. The curved interconnecting elements are flexible enough to allow each tooth-clasping element to remain firmly engaged in place. The flexible properties of the interconnecting elements are controlled by the choice of materials, by the cross-section of the interconnecting elements, and by the shape of the interconnecting elements. The shape chosen in most of the embodiments of the interconnecting element in the present invention is a small radius loop configuration, where the radius of the loop is preferably about half of the width of the tooth.